security update Linux-package glibc ?

Security fixes are already included from Debian. Proxmox is OpenSource and all packages are available through their testing repositoriy. This repository contain then bleeding edge of their git repositories and can contain errors, bugs and stuff like that. After a while (testing) the files are released into the "production" and subscription-only repositories.

If you can live with the possibility that an update will break your stuff, you can use the testing repository.

For further information, please see the following page:
https://pve.proxmox.com/wiki/Package_repositories#Proxmox_VE_No-Subscription_Repository